A vertical furnace, such as a chemical vapor deposition (CVD) process chamber, is used for depositing various films, such as polysilicon, silicon oxide, and silicon nitride films on a large number of semiconductor wafers at one time. To form a thin film with a uniform thickness on many semiconductor wafers at one time in the vertical furnace, a uniform flow of well-mixed precursors (reactive gas) must be produced in a heat treatment tube. However, most of reactive gas inlets of the vertical furnaces are designed near or at a bottom end of the heat treatment tube, and inadequate mixing of precursors occurs in the heat treatment tube, thus resulting in poor uniformity of the thin film formed on the semiconductor wafers.